'\" t
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH mq_open 3 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
mq_open \- open a message queue
.SH LIBRARY
Real-time library
.RI ( librt ", " \-lrt )
.SH SYNOPSIS
.nf
.BR "#include <fcntl.h>" "           /* For O_* constants */"
.BR "#include <sys/stat.h>" "        /* For mode constants */"
.B #include <mqueue.h>
.P
.BI "mqd_t mq_open(const char *" name ", int " oflag );
.BI "mqd_t mq_open(const char *" name ", int " oflag ", mode_t " mode ,
.BI "              struct mq_attr *" attr );
.fi
.SH DESCRIPTION
.BR mq_open ()
creates a new POSIX message queue or opens an existing queue.
The queue is identified by
.IR name .
For details of the construction of
.IR name ,
see
.BR mq_overview (7).
.P
The
.I oflag
argument specifies flags that control the operation of the call.
(Definitions of the flags values can be obtained by including
.IR <fcntl.h> .)
Exactly one of the following must be specified in
.IR oflag :
.TP
.B O_RDONLY
Open the queue to receive messages only.
.TP
.B O_WRONLY
Open the queue to send messages only.
.TP
.B O_RDWR
Open the queue to both send and receive messages.
.P
Zero or more of the following flags can additionally be
.IR OR ed
in
.IR oflag :
.TP
.BR O_CLOEXEC " (since Linux 2.6.26)"
.\" commit 269f21344b23e552c21c9e2d7ca258479dcd7a0a
Set the close-on-exec flag for the message queue descriptor.
See
.BR open (2)
for a discussion of why this flag is useful.
.TP
.B O_CREAT
Create the message queue if it does not exist.
The owner (user ID) of the message queue is set to the effective
user ID of the calling process.
The group ownership (group ID) is set to the effective group ID
of the calling process.
.\" In reality the filesystem IDs are used on Linux.
.TP
.B O_EXCL
If
.B O_CREAT
was specified in
.IR oflag ,
and a queue with the given
.I name
already exists, then fail with the error
.BR EEXIST .
.TP
.B O_NONBLOCK
Open the queue in nonblocking mode.
In circumstances where
.BR mq_receive (3)
and
.BR mq_send (3)
would normally block, these functions instead fail with the error
.BR EAGAIN .
.P
If
.B O_CREAT
is specified in
.IR oflag ,
then two additional arguments must be supplied.
The
.I mode
argument specifies the permissions to be placed on the new queue,
as for
.BR open (2).
(Symbolic definitions for the permissions bits can be obtained by including
.IR <sys/stat.h> .)
The permissions settings are masked against the process umask.
.P
The fields of the
.I struct mq_attr
pointed to
.I attr
specify the maximum number of messages and
the maximum size of messages that the queue will allow.
This structure is defined as follows:
.P
.in +4n
.EX
struct mq_attr {
    long mq_flags;       /* Flags (ignored for mq_open()) */
    long mq_maxmsg;      /* Max. # of messages on queue */
    long mq_msgsize;     /* Max. message size (bytes) */
    long mq_curmsgs;     /* # of messages currently in queue
                            (ignored for mq_open()) */
};
.EE
.in
.P
Only the
.I mq_maxmsg
and
.I mq_msgsize
fields are employed when calling
.BR mq_open ();
the values in the remaining fields are ignored.
.P
If
.I attr
is NULL, then the queue is created with implementation-defined
default attributes.
Since Linux 3.5, two
.I /proc
files can be used to control these defaults; see
.BR mq_overview (7)
for details.
.SH RETURN VALUE
On success,
.BR mq_open ()
returns a message queue descriptor for use by other
message queue functions.
On error,
.BR mq_open ()
returns
.IR "(mqd_t)\ \-1",
with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EACCES
The queue exists, but the caller does not have permission to
open it in the specified mode.
.TP
.B EACCES
.I name
contained more than one slash.
.\" Note that this isn't consistent with the same case for sem_open()
.TP
.B EEXIST
Both
.B O_CREAT
and
.B O_EXCL
were specified in
.IR oflag ,
but a queue with this
.I name
already exists.
.TP
.B EINVAL
.\" glibc checks whether the name starts with a "/" and if not,
.\" gives this error
.I name
doesn't follow the format in
.BR mq_overview (7).
.TP
.B EINVAL
.B O_CREAT
was specified in
.IR oflag ,
and
.I attr
was not NULL, but
.I attr\->mq_maxmsg
or
.I attr\->mq_msqsize
was invalid.
Both of these fields must be greater than zero.
In a process that is unprivileged (does not have the
.B CAP_SYS_RESOURCE
capability),
.I attr\->mq_maxmsg
must be less than or equal to the
.I msg_max
limit, and
.I attr\->mq_msgsize
must be less than or equal to the
.I msgsize_max
limit.
In addition, even in a privileged process,
.I attr\->mq_maxmsg
cannot exceed the
.B HARD_MAX
limit.
(See
.BR mq_overview (7)
for details of these limits.)
.TP
.B EMFILE
The per-process limit on the number of open file
and message queue descriptors has been reached
(see the description of
.B RLIMIT_NOFILE
in
.BR getrlimit (2)).
.TP
.B ENAMETOOLONG
.I name
was too long.
.TP
.B ENFILE
The system-wide limit on the total number of open files
and message queues has been reached.
.TP
.B ENOENT
The
.B O_CREAT
flag was not specified in
.IR oflag ,
and no queue with this
.I name
exists.
.TP
.B ENOENT
.I name
was just "/" followed by no other characters.
.\" Note that this isn't consistent with the same case for sem_open()
.TP
.B ENOMEM
Insufficient memory.
.TP
.B ENOSPC
Insufficient space for the creation of a new message queue.
This probably occurred because the
.I queues_max
limit was encountered; see
.BR mq_overview (7).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR mq_open ()
T}	Thread safety	MT-Safe
.TE
.SH VERSIONS
.SS C library/kernel differences
The
.BR mq_open ()
library function is implemented on top of a system call of the same name.
The library function performs the check that the
.I name
starts with a slash (/), giving the
.B EINVAL
error if it does not.
The kernel system call expects
.I name
to contain no preceding slash,
so the C library function passes
.I name
without the preceding slash (i.e.,
.IR name+1 )
to the system call.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH BUGS
Before Linux 2.6.14,
the process umask was not applied to the permissions specified in
.IR mode .
.SH SEE ALSO
.BR mq_close (3),
.BR mq_getattr (3),
.BR mq_notify (3),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
.BR mq_overview (7)
